Up to now, as disclosed in Patent Literature 1, a method of manufacturing an SOI wafer in which two silicon wafers are bonded together has been known. First, an oxide film is formed on both of the silicon wafers. Then, the silicon wafers are bonded together through the oxide film. The silicon wafers have been bonded together, and thereafter heated. A heating temperature is set to 1200° C. or more in order to secure a bonding strength. With the above process, the SOI wafer can be manufactured.
In the above method, a water may be generated at a bonding interface between the silicon wafers due to heating. If the water is generated at the interface, a bonding strength may be reduced. On the other hand, it is known that a film thickness of the oxide film in one silicon wafer is set to 50 nm or more in order to prevent the reduction in the bonding strength. With an increase in the film thickness of the oxide film, water generated by heating is absorbed by the oxide film, and the reduction in the bonding strength can be prevented.
However, similarly in the above method, in order to secure the bonding strength, there is a need to heat the silicon wafer at high temperature. Since a wiring made of aluminum or the like and an impurity diffusion layer are formed on the silicon wafer, a maximum temperature at which the silicon wafer can be heated is determined according to a heat resistant temperature of the wiring and a temperature at which the impurity diffusion layer diffuses. According to the maximum temperature, in general, the temperature at which the silicon wafer can be heated is set to 800° C. or less. Therefore, in a method of heating at the high temperature described above, the SOI wafer cannot be manufactured.
On the other hand, it is conceivable to manufacture the SOI wafer through a method in which the oxide film is formed on one surface of the silicon wafer and no oxide film is formed on the other surface. In more detail, the silicon wafers are bonded to each other due to Si—SiO2 bonding to manufacture the SOI wafer. In the bonding method, the bonding strength is secured while the heating temperature kept low. However, in the bonding method, when the silicon wafers are bonded together, no oxide films can be formed on both of the silicon wafer surfaces.
By the way, it is known that with the formation of the oxide film on the silicon wafer surface, out-diffusion can be prevented, and an interface level can be reduced. On the other hand, in the above configuration, since the oxide film cannot be formed on both of the silicon wafer surfaces, the out-diffusion may occur, and the interface level may become large. According to the above configuration, a leakage current may occur in the SOI wafer. Further, in the above-described configuration in which the oxide film cannot be formed on both of the silicon wafer surfaces, the degree of freedom in designing the SOI wafer may decrease.